An electronic device system in which a computer is mounted on an expansion unit is disclosed in Patent Literature 1. Patent Literature 1 discloses an electronic device system that includes a computer having a housing with a modem built therein and a first extension connector disposed on a rear surface of the housing, and an expansion unit having a placement surface on which the computer is to be mounted on a removable manner and a second extension connector that is formed on the placement surface and to which the first extension connector is to be connected. In addition, the computer includes a first relay connector electrically connected to the modem, and the expansion unit includes a connector for communication to which a communication line is to be connected, and a second relay connector connected to the connector for communication. When the computer is mounted on the placement surface, the second relay connector is electrically connected, on the same lateral surface of the second extension connector, to the first relay connector that is separate from the second extension connector.